Metabolome and Transcriptome Analysis Reveals the Regulatory Effect of Magnesium Treatment on EGCG Biosynthesis in Tea Shoots (Camellia sinensis)

Epigallocatechin-3-O-gallate (EGCG) is an important ingredient that indicates tea quality and has healthcare functions. Magnesium nutrition can improve the quality and yield of tea plants, but its regulatory role in the biosynthesis of EGCG in tea plants has not been clarified. Herein, we performed a comprehensive analysis of the metabolomics and transcriptomics of the shoots of ‘Huangdan’ at five magnesium concentrations: L1-L5 (0, 0.15, 0.45, 0.6, and 0.9 mmol/L mg2+, respectively). The results showed that the EGCG content of tea shoots treated with low magnesium concentrations was higher compared to those treated with high magnesium concentrations. The contents of related metabolites such as p-coumaric acid and cyanide in the EGCG synthesis pathway increased in the L4 and L5 treatment groups, while those of dihydroquercetin, dinnamic acid, and epicatechin increased significantly in the L2 and L3 treatment groups. Under the influence of magnesium treatment, the biosynthesis of EGCG was affected by a series of structural genes: CsPAL (HD.01G0005520), HD.02G0024350), Cs4CL (HD.15G0008250, HD.13G0010220), CsDFR (HD.04G0026220), CsANS(HD.12G0016700) with CsaroDE (HD.03G0002480)-positive regulation, and CsPAL (HD.13G0009900, HD.06G0008610), CsC4H (HD.06G0017130), Cs4CL (HD.02G0027390, HD.04G0003270), CsCHS (HD.10G0022640), CsCHI (HD.01G0011100), CsF3′H (HD.15G0015490), CsF3′5′H (HD.13G0004300), CsANS (HD.07G0023630), and Csaro B (HD.01G0028400) with CsSCPL (HD.01G0041070)-negative regulation. Transcription factors MYB 44 and WRKY 17 may play a key role in EGCG biosynthesis, which is significantly induced by magnesium nutrition in tea tree shoots. This study elucidates the effect of magnesium nutrition on EGCG biosynthesis in tea plants and provides key candidate transcription factors to provide a reference for further research on high-EGCG tea varieties to improve tea quality.